1. Field of Invention
The present invention relates to a hot plate cooling system. More particularly, the present invention relates to the cooling system inside a hot plate to be used mainly in rapid thermal processing, baking and chemical vapor deposition for rapidly cooling the hot plate after heating.
2. Description of Related Art
In the manufacturing of integrated circuits, a silicon wafer has to go through a series of thermal operations. Besides growing thin films, other processes such as diffusion and photoresist baking also require heating as well. In addition, semiconductor fabrication also involves many thermal treatments including annealing or heat reflow operation. In general, thermal oxidation, annealing, heat reflow, diffusion and material curing operations are conducted in a furnace. However, photoresist baking is usually conducted on a hot plate instead of inside a furnace. Photoresist baking is a low temperature (at about 200° C.) heat solidification process. To facilitate incorporation with other photolithographic processes and to prevent uneven vaporization of solvent inside the photoresist, a soft baking, post exposure baking and hard baking are all conducted using a hot plate.
FIGS. 1A and 1B show respectively the top view and the side view of a conventional hot plate. As shown in FIGS. 1A and 1B, a conventional hot plate uses natural convection current for cooling. Natural convection cooling is able to achieve a cooling rate of about 0.8° C. per minute on average. Therefore, lowering the hot plate by 40° C. requires about 50 minutes.
After treating a batch of wafers, the hot plate needs to be cooled by natural convection because temperature of the hot plate is not too high. However, natural convection cooling is relatively slow process and hence the next batch of wafers has to be put aside for the long wait while the hot plate cools down.